Simple Calendar - A simple calendar with events (optional recurring and reminders), week numbers and a customizable...
FullCalendar - FullCalendar is an open-source JavaScript event calendar.
Etar - Material design open source calendar for Android.
Dudle - Dudle is an online scheduling application, which is free and open source.
Google Calendar - Spend less time managing your day & more time enjoying it
Mobile-Calendar - Reservation management system. If you rent accommodation, the mobile-calendar app was made for you!